1. Field of the Invention
The invention relates to a process for hydrogenating an aromatic amine that has at least one amino group bound to an aromatic nucleus, wherein the hydrogenation with hydrogen takes place in the presence of a supported catalyst containing ruthenium as an active metal.
2. Discussion of the Background
Cycloaliphatic amines obtained in the catalytic hydrogenation of aromatic amines, such as unsubstituted or substituted cyclohexyl amines and dicyclohexyl amines, are useful in the preparation of polyamide and polyurethane resins, as hardeners for epoxy resins and also as raw materials for the preparation of plastic additives, rubber additives, and corrosion inhibitors.
It is known to prepare cycloaliphatic amines containing one or more amino groups by catalytic hydrogenation of the corresponding mononuclear or polynuclear aromatic amines containing one or more amino groups and optionally further substituents. Such amines are hydrogenated to form the corresponding cycloaliphatic amines, often using supported catalysts.
Thus, in the process described in U.S. Pat. No. 2,606,925, bis(4-minophenyl)methane (hereinafter, methylenedianiline or MDA) is hydrogenated in the presence of a ruthenium supported catalyst, active carbon, aluminum oxide and kieselguhr as support, to form bis(4-aminocyclohexyl) methane (hereinafter bis(p-aminocyclohexyl)methane or PACM). In this process, PACM takes the form of the cis-cis, cis-trans and trans-trans isomers. Hydrogenation temperatures above 150° C. or prolonged reaction times during the hydrogenation result in an increased proportion of the trans-trans isomers. No indications can be found in U.S. Pat. No. 2,606,925 whether and in what way the selection of the catalyst or catalyst support influences the isomer distribution.
In an effort to obtain the thermodynamically more stable trans-trans isomer of PACM, the hydrogenation described in U.S. Pat. Nos. 3,155,724 and 3,347,917 is performed using a ruthenium supported catalyst in the presence of ammonia. The generic hydrogenation can also be improved, in accordance with U.S. Pat. No. 3,914,307, in that the reaction mixture to be hydrogenated additionally contains a polyheterocyclic amine as cocatalyst. No indications can be found in any of these three patents about the isomeric ratio in the hydrogenation of polynuclear aromatic amines such as MDA to form PACM or any influence of the properties of the support material.
U.S. Pat. No. 5,360,934 discloses a generic method, but a rhodium-containing supported catalyst is used. Ruthenium may also be present as active metal. According to this reference, the catalyst activity depends to an appreciable extent on the modification of the aluminum oxide used as support. According to this reference, catalysts containing delta-, theta- and kapa-aluminum oxide as support material are more active than a catalyst containing commercial gamma-aluminum oxide as support material.
In accordance with EP 0 066 211 A1, dianilinomethane can be converted into PACM having a trans-trans isomeric PACM component in the range from 15 to 40 wt. %, in particular 18.5 to 23.5 wt. % by performing the hydrogenation in the presence of a support-free ruthenium catalyst. This process has the disadvantages of high hydrogenation pressure, high reaction temperature, and the greater effort expended on separating the ruthenium catalyst from the reaction mixture.
In accordance with EP patent 0 324 190, PACM containing the above-mentioned low trans-trans isomeric component can be obtained by hydrogenating MDA in the presence of a support-bound ruthenium catalyst. The hydrogenation takes place at 100 to 190° C. and a pressure of 5 to 35 MPa, in which process, although the temperature was in the lower temperature range in the exemplary embodiments, the hydrogen pressure at 30 MPa was in the upper part of the above-mentioned pressure range. The support material of the catalyst used in this process is characterized by a BET surface area range of 70 to 280 m2/g and a mean pore diameter dp of 1 to 32 nm; the penetration depth of the ruthenium is at least 50 μm, in particular 100 to 300 μm; and the ruthenium content is specified as 0.1 to 5 wt. %, in particular 0.5 to 3 wt. %. A disadvantage of this process however is that a high hydrogen hydrogenation pressure is still required in practice.
In accordance with EP 0 813 906 A2, organic compounds, including aromatic compounds in which at least one amino group is bound to an aromatic nucleus, can be hydrogenated using a ruthenium supported catalyst. In addition to ruthenium, the catalyst may contain, as active metal, other metals from the first, seventh or eighth subgroup of the periodic system. In contrast to the support material of EP patent 0 324 190 mentioned above, the support material has, in EP 0813906 A2, a BET surface area of not more than 30 m2/g and a mean pore diameter of at least 50 nm. The catalyst used in this case is also characterized by a ratio of the surface area of the active metal and the surface area of the catalyst support of less than 0.05. The macroporous support materials having a mean pore diameter of preferably 500 nm to approximately 50 μm are preferably aluminum oxide and zirconium oxide. Details on the hydrogenation of MDA to form PACM are not found in this document, however. In particular, the hydrogenation of aromatic compounds such as 4-alkalyl-substituted phenols results predominantly in trans cycloaliphatic compounds.
A similar process to that of EP 0 813 906 A2 is disclosed in EP 0 814 098 A2: the support materials for the support-bound ruthenium hydrogenation catalysts for hydrogenating aromatic amines to cycloaliphatic amines are those materials in which 10 to 50% of the pore volume is formed by macropores having a pore diameter in the range from 50 nm to 10,000nm and 50 to 90% is formed by mesopores having a pore diameter in the range from 2 to 50nm. The BET surface area of the support is specified as 50 to 500 m2/g, in particular 200 to 350 m2/g. The ratio of the surface of the active metal and of the support is said to be less than 0.3, in particular less than 0.1. Neither the activity of such catalysts nor the isomeric ratio in the case of the hydrogenation of MDA to form PACM are described in this reference. However, reference is also made here to the predominant formation of the trans isomers with reference to the hydrogenation of 4-substituted phenols.
EP 0 873 300 B1 proposes carrying out the hydrogenation of aromatic amines such as bis(p-aminophenyl)methane, with a catalyst containing support-bound ruthenium as active metal, the support having a mean pore diameter of at least 0.1 μm, in particular at least 0.5 μm and a surface area of not more than 15 m2/g, preferably 0.05 to 5 m2/g. Cycloaliphatic amines can be obtained by this process with high selectivity and without the formation of deamination products or partially hydrogenated dimerization products. Trans-trans isomeric components are not disclosed.
As is disclosed by G. F. Allen (Chem. Ind. (Dekker) (1988), 33 Catal. Org. Reakt., 323-338), the trans-trans isomeric PACM component increases with increasing conversion of metylhenediamine. The two above-mentioned processes relate, however, only to obtaining a high conversion.
EP 0 639 403 A2 discloses that bis(p-aminocyclohexyl)methane can be produced by hydrogenation of methylenedianiline with a ruthenium-containing or rhodium-containing supported catalyst, the layer thickness of the active metals on the support being 5 to 150 μm, preferably 10 to 80 μm. The support material is a calcined and superficially rehydrated transition argillaceous earth having a specified pH. The support has a BET surface area of at least 70 m2/g and an open porosity of at least 0.1 ml/g. Pore distribution is not disclosed. An advantage of this process is that, even after a prolonged operating time, the proportion of trans-trans PACM isomer is in the range from about 20 to 25%. Disadvantages of this process are, however, the increased effort in establishing an equilibrium pH, the high expenditure on equipment required to apply the high hydrogen pressure (300 bar) mentioned in the examples, and the limited selection of supports.
In accordance with the process disclosed in DE 199 42 813, PACM with a low trans-trans isomer component can be obtained in an advantageous way at a moderate temperature of 50 to below 130° C. and a moderate hydrogen pressure of 3 to 10 MPa using a ruthenium supported catalyst bound to titanium dioxide or aluminum oxide. The specific surface area of the titanium dioxide used particularly preferably is in the range from greater than 20 m2/g to less than 70 m2/g. A disadvantage of this process, however, is the high ruthenium content of the supported catalyst required. Pore structure/distribution is not disclosed.